1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for measuring a source signature.
2. Discussion of the Background
In seismic surveys used in the gas and oil industry to investigate underground structures, one or more individual seismic sources are actuated to emit seismic waves propagating downward through the investigated structure, where the seismic waves are reflected by geological interfaces that act as partial reflectors. The reflected seismic waves are detected by sensors (generally referred to as “receivers”) recording seismic data, which is processed and converted into structural information.
When individual seismic sources are actuated according to an actuation sequence, they emit seismic waves over a defined period of time (in the range of a fraction of a second to a few seconds) within a known range of frequencies (e.g., 10-150 Hz). The emitted amplitude is frequency-dependent. Close to the individual sources (i.e., near-field), the amplitude-versus-frequency shape varies as the seismic waves propagate spatially. However, far from the individual sources (far-field), the amplitude-versus-frequency shape ceases to vary as the seismic energy propagates spatially, and the amplitude decreases inversely proportional to the distance from the individual sources. The substantially constant amplitude-versus-frequency shape far enough from the individual sources is known as the source signature. Note that while the source signature is a two-dimensional amplitude-versus-frequency shape for impulse-type seismic sources (e.g., air-guns), it has an additional time dimension for seismic sources emitting energy during a longer period (e.g., vibrating sources). An arrangement of plural individual seismic sources is known as source array.
Knowledge of the source signature is desirable to better identify geological features when seismic data is processed. In mathematical terms, seismic data collected by a seismic receiver is a convolution of the source signature and the underground structure's response to the seismic waves emitted by the source. The more accurate the knowledge of the source signature, the more accurately the underground structure's response may be recovered from the acquired seismic data to generate an image thereof.
The source signature depends on characteristics of individual sources (e.g., type, volume, initial individual amplitude, etc.), their geometrical arrangement (e.g., inline distances between the individual sources in a seismic sub-array, distance between the sub-arrays, depths of the individual sources, etc.), the actuation sequence of the individual sources, and environmental conditions (e.g., weather, currents, etc.). Some of these parameters, such as the characteristics of individual sources or the actuation sequence, can be reliably reproduced from shot to shot. Some other parameters, such as environmental conditions, the geometrical arrangement, or the sources' operational states, may result in unpredictable and unavoidable variations. Thus, the source signature may vary from shot to shot.
Conventionally, as illustrated in FIG. 1, a vessel 110 tows (in T direction) one or more hydrophones 120 at more than 100 m under a source array 130 to measure the source signature. Source array 130 and hydrophones 120 may easily drift laterally relative to one another, compromising detection of the direct waves from the source array. Additionally, towing hydrophones in this manner is challenging due to the relatively great towing depth and the risk of the cable connecting the hydrophones to the vessel tangling.
Another conventional method for detecting a source signature employs the setup illustrated in FIG. 2 (a bird's-eye view). A buoy with hydrophones attached is maintained in a stationary position under point 210, at about 100 m deeper than the source array's towing depth. A vessel 220 tows a source array 230 on trajectory 240 to bring the source array 230 above the hydrophones. Towed source array 230 is actuated when above the hydrophones so that the hydrophones detect the seismic waves traveling directly from the source array to the hydrophones. This method is time-consuming and cumbersome because it is difficult to fire the source array 230 exactly vertical above the hydrophones, which may drift. Although the method illustrated in FIG. 2 provides a direct measurement of the source signature, the method is not usable during regular seismic data acquisition.
Another conventional method estimates the source signature shot-by-shot based on a model starting from the directly-measured source signature and using near-field measurements of the emitted seismic waves. The near-field measurements are acquired using near-field sensors placed in proximity to each of the individual sources. The main disadvantage of this approach is that during direct measurement, the source array and seismic detectors may easily drift apart from one another causing unreliable detection. This problem is so prevalent that software simulations are preferred to the estimates of the source signature. However, a simulated result is only as good as the phenomenological model implemented in the software.
In the context of a complex reality and increasing variety of individual source arrays, it would be desirable to provide devices, systems, and methods that reliably measure a source signature shot-by-shot while acquiring seismic data.